The present invention relates to a voltage-to-current conversion circuit and, more particularly, to such a circuit as utilizing an insulated gate field effect (MOS) transistors.
A voltage-to-current conversion circuit is essential to a signal processing circuit handling an analog signal and thus widely employed therein. This circuit fundamentally uses a current mirror circuit composed of transistors there are two types in such a circuit, a bipolar type and a MOS type.
The voltage-to-current conversion circuit of the bipolar type is typically disclosed in Japanese. Laid-open patent No. Sho 59-221014. The basic circuit construction thereof is shown in FIG. 3 attached herewith (and corresponds to FIG. 5 of the above patent No.). Specifically, this circuit includes a current mirror circuit composed of PNP bipolar transistors 20 and 22, an input resistor 60, a constant current source composed of an NPN bipolar transistor 21, a resistor 61 and a constant voltage source 80, and an operational amplifier 40 which are connected as shown. The amplifier 40 cooperates with the transistor 20 to form a strong negative feedback loop. Accordingly, the current flowing through the resistor 60 in response to an input signal voltage supplied to an input terminal 90 is determined by the resistance value of the resistor 60 and the voltage amplitude of the input signal voltage. This current is supplied to the transistor 20 and thus derived from an output terminal 91 through the transistor 22 as the mirror current. The voltage-to-current conversion is thus attained. This circuit can handle a relatively large input voltage and further operate under a relatively low power voltage (1.8 V).
The conversion circuit of the MOS type shown in FIG. 4 is well in the art, on the other hand. This circuit also includes a current mirror circuit composed of P-channel MOS transistors 30 and 32 and an operational amplifier 41. However, this circuit converts the input signal voltage into the current by an N-channel MOS transistor 31, a resistor 62 and the amplifier 41 and then produces the converted current through the current mirror circuit. Specifically, the negative feedback loop composed of the transistor 31 and the amplifier 41 causes the input signal voltage to appear across the resistor 62. The current determined by the resistance value of the resistor 62 and the voltage amplitude is thereby derived from the output terminal 92 through the transistor 31 and the current mirror circuit. The present circuit is advantageous to have a high input impedance due to the operational amplifier 41 receiving the input voltage.
Referring again to FIG. 3, the bipolar transistors constituting this circuit can be replaced by MOS transistors. However, the circuit thus replaced requires a power supply voltage larger than that applied to the circuit of FIG. 3. This is because the threshold voltage and the saturated voltage of the MOS transistor are higher than those of the bipolar transistor. Therefore, the conversion circuit of FIG. 3 is preferable in use in a low power voltage system. However, the input impedance is determined by the resistor 60 and thus cannot be enhanced. A source follower circuit is, for example, required to enhance the input impedance, so that the amplitude of the input voltage is restricted.
On the other hand, the circuit shown in FIG. 4 has a high input impedance, as described hereinbefore. However, the source voltage of the transistor 31 is increased in accordance with increase in voltage of the input signal. For this reason, a relatively high drain voltage is needed. The power supply voltage is required to be high accordingly, or the input signal voltage is restricted.